Risk of Kidney Stone Formation During Pregnancy for Women with History of Calcium Oxalate Stones
The risk of kidney stone formation for a woman with a history of calcium oxalate stones is increased during the second and third trimesters of pregnancy, peaking in the third trimester and immediately postpartum. 1
Physiological Changes During Pregnancy Affecting Stone Risk
Pregnancy induces several changes that influence kidney stone formation:
- Hypercalciuria: Urinary calcium excretion increases significantly during pregnancy (251 ± 127 mg/day during pregnancy vs. 121 ± 67 mg/day postpartum) 2
- Increased urine saturation: Higher levels of calcium oxalate saturation (3.0 ± 1.1 during pregnancy vs. 2.1 ± 1.0 postpartum) and brushite saturation (1.9 ± 1.1 vs. 1.2 ± 0.9) 2
- Stone composition shift: Calcium phosphate (hydroxyapatite) stones become more common during pregnancy (74% of stones) compared to calcium oxalate stones (26%) 3
Trimester-Specific Risk Pattern
Research demonstrates a clear pattern of increasing risk throughout pregnancy:
- First trimester: Similar risk to non-pregnant state (OR 0.92) 1
- Second trimester: Risk begins to increase (OR 2.00) 1
- Third trimester: Risk continues to increase (OR 2.69) 1
- 0-3 months postpartum: Peak risk period (OR 3.53) 1
- By 1 year postpartum: Risk returns to baseline 1
Protective Factors During Pregnancy
Despite the increased risk factors, some protective mechanisms exist:
- Increased urinary thiosulfate: Levels increase to approximately 36,38, and 40 μM/24 hour across the three trimesters, returning to normal after delivery 4
- This gestational hyperthiosulfaturia may provide some protection against stone formation despite hypercalciuria 4
Preventive Measures for Pregnant Women with Stone History
The American Urological Association recommends the following preventive measures for women with stone history who are pregnant or planning pregnancy:
- Maintain adequate hydration: Achieve urine volume of at least 2.5 liters daily 5, 6
- Normal dietary calcium: Consume 1,000-1,200 mg/day of dietary calcium 5, 6
- Limit sodium intake: Target 100 mEq (2,300 mg) daily to reduce urinary calcium excretion 5, 6
- Limit oxalate-rich foods: Particularly important for those with history of hyperoxaluria 5, 6
- Timing of calcium intake: Consume calcium from foods primarily at meals to enhance gastrointestinal binding of oxalate 5
Monitoring and Management During Pregnancy
For women with history of stones who become pregnant:
- Regular ultrasound monitoring: Every 2-4 weeks to monitor for hydronephrosis 6
- First-line imaging: Renal ultrasonography with color Doppler if symptoms develop 6
- Conservative management: Hydration and appropriate pain control for uncomplicated stones 6
- Intervention indications: Failed conservative management, severe infection/pyonephrosis, or solitary kidney with significant obstruction 6
Conclusion
For a 35-year-old woman with history of calcium oxalate stones who wants to become pregnant, the risk of stone formation will be increased during the second and third trimesters, with the highest risk in the third trimester and immediately postpartum. Implementing preventive measures before and during pregnancy is crucial to minimize this risk.